Flathead grey muller (Mugil cephalus L. 1758) are commonly found in tropical and temperate estuaries. In spite of their ecological and economical importance, little is known about the effects of salinity on growth performance and habitat selection. Laboratory experiments revealed that the metabolic rate of young specimens was negatively affected by high salinity levels and that an improved growth performance was achieved in fresh water and oligohaline water. A stratified study on microhabitat use, carried out on the island of Minorca (Balearic archipelago), demonstrated that juvenile specimens, shorter than 200 mm (total length), concentrated all year round in fresh water or oligohaline sites. Mesohaline areas were usually avoided, except in summer. Immature fish, with a total length between 201 and 300 mm, show a similar pattern although in some seasons avoided freshwater sires. The habitat selection pattern of adults, i.e., fish longer than 301 mm, changed seasonally due to their offshore migration during the spawning season (from late summer to early winter). However, they usually showed a greater preference for polyhaline areas and strongly avoided freshwater sites. Euhaline areas were also avoided in autumn and summer. These results suggest that the young of this species are highly dependent on areas of low salinity and any factor which reduces the availability of such areas will, in turn, affect their fishery. Adults depend on polyhaline areas, although the avoidance of freshwater areas might be due to their shallowness. The situation might be different in some areas of the Indo-Pacific. These differences are discussed. (C) 2000 Academic Press.
